Ecophysiological and molecular involvement of extracellular-regulated protein kinases (ERK 1/2) in the response of Dunaliella viridis to heat stress

Abstract

The eukaryotic green microalga Dunaliella viridis has shown an outstanding capacity to face a broad range of environmental stressors such as, high irradiance, UV radiation, salinity, and temperature, among others. The lack of a rigid cell wall, as well as, its unique ability to respond, adapt and grow under stressful conditions makes Dunaliella an excellent model to study stress signal transduction in eukaryotic unicellular organisms. Mitogen-activated protein kinases (MAPKs) are highly conserved serine/threonine kinases that convert extracellular stimuli into a wide variety of responses at both cellular and nuclear levels. In eukaryotic cells, MAPKs are involved in both environmental stress responses (JNK and p38 pathways) and cell proliferation and differentiation (ERK pathway) through protein kinase cascades. Previously, two different ERK proteins, ERK1 and ERK2 (ERK1/2) have been identified in D. viridis. Based on the non-direct implication of the ERK-like proteins in the acclimatization process against environmental stress proven by the specific blockade of the cascades, different short-term heat-shock (SH) experiments have been conducted in this work to uncover molecular mechanisms underlying the role of ERK1/2 in heat stress. The evolution of the maximum quantum yield of D. viridis after non-lethal SH, together with protein immunedetection by ERK1/2 western blotting and qRT-PCR experiments revealed that the ERK1/2 proteins are not directly involved in the response to heat stress, and that they are rapidly deactivated after stress, leading to a transient inhibition of cell division. Behaviour of MAPK-like proteins in algae is largely unknown at present. The analysis of their mechanism of action, as well as their function in this model microalga, will allow us to decipher the fate of unicellular eukaryotic organisms in aquatic ecosystems subjected to environmental stress derived from the conditions prevailing within a framework of global climate change.